Granted my son is a 12u but his team recently did a 5 week program based on House's and his velocity went up 7 mph . The lower body worked seemed to really be the key. I think that for young kids they are using muscles that they have never used/developed before. Not sure about the older boys probably more about a few mph and arm care. The program is phenomenal for arm care. I am skeptical about the weighted ball programs but just like TCB heavy balls its something you can do in tight space.
Personally I love them to do long toss and long toss with a football as much as they can in the off season. Something about throwing the football that really seems to help.